1. Field of the Invention
The present invention relates to a matrix type super twisted nematic (STN) liquid crystal display device and a method for driving the device. The device and method are used in office automation equipment such as a personal computer and word processor, multi-media personal digital assistants, audio and video equipment, game machines, and the like. More particularly, the present invention relates to a liquid crystal display device and a driving method therefor which can improve display quality.
2. Description of the Related Art
Conventional STN liquid crystal display (LCD) devices have a problem that as display capacity, such as liquid crystal capacity is increased, display irregularity depending on display patterns emerges, leading to a significant decrease in display quality. Such display irregularity is called crosstalk.
An example of such crosstalk is one caused by induced distortion of scanning voltage (hereinafter referred to as “induced distortion crosstalk”). Specifically, when the waveforms of signal voltages applied to a number of column electrodes are simultaneously changed, a high level of induced distortion occurs in scanning voltage, so that an effective voltage value applied to each pixel is increased or decreased to be shifted from an intended effective voltage value.
FIG. 14A is a diagram used to briefly explain induced distortion crosstalk, showing a liquid crystal panel 140 including row electrodes Y1 through Y4 and column electrodes X1 through X4. When signal voltages SG1 through SG4 having waveforms shown in FIG. 14B are applied to the column electrodes X1 through X4 of FIG. 14A, induced distortion S1 through S4 occurs in the scanning voltage on the row electrode Y1 as shown in FIG. 14C. Similar induced distortion occurs in the scanning voltage on the row electrodes Y2 and Y3.
The magnitudes of induced distortion S1 through S4 occurring in the scanning voltage on the row electrode Y1 vary depending on the number of signal voltages SG1 through SG4 which are simultaneously changed. The more signal voltages simultaneously changed in the same direction, the larger the magnitudes. As shown in FIGS. 14B and 14C, when signal voltages which are changed in opposite directions cancel one another, smaller induced distortion occurs in a row electrode (S3 in FIG. 14C) as compared to when signal voltages are changed in the same direction (S1, S2 and S4 in FIG. 14C).
To solve the above-described problems, for example, Japanese Laid-Open Publication No. 6-27899 proposes a first conventional technique in which a change in voltage on a row electrode is detected and, in response to the change, a voltage on a column electrode is adjusted so that display irregularity is overcome.
Alternatively, Japanese Laid-Open Publication No. 11-84342 proposes a second conventional technique in which display data D(n) on an nth scanning line is compared to display data D(n−1) on an (n−1)th scanning line so that {M(HL)−M(LH)} is calculated where M(HL) is the number of data which transit from an H (High) level to an L (Low) level and M(LH) is the number of data which transit from the L level to the H level, and then a correction voltage having a magnitude and a direction corresponding to the calculation result is added from a column electrode to a signal voltage so as to correct the signal voltage.
Further, Japanese Laid-Open Publication No. 11-52326 proposes a third conventional technique in which a correction period equal to one or two horizontal scanning periods is inserted every a predetermined number of horizontal scanning periods.
Another type of crosstalk is now described. When the signal voltages SG1 through SG4 applied to the column electrodes X1 through X4 becomes “blunt” with respect to ideal waveforms due to resistance components of electrodes or capacity components of a liquid crystal layer in a liquid crystal panel, crosstalk (hereinafter referred to as “blunt waveform crosstalk”) occurs.
There is also a phenomenon where there is a difference in brightness in the lateral direction of a screen independent of display patterns (hereinafter referred to as the “gradation phenomenon”). This is because a decrease in voltage occurs along a row electrode due to a resistance component of the row electrode and therefore a difference in an effective voltage value applied to a liquid crystal layer develops with respect to the lateral direction along the row electrode.
In fact, the above-described induced distortion crosstalk varies in a lateral direction along a row electrode due to both the capacity of a liquid crystal layer and the resistance of a row electrode.
FIG. 15 shows a difference in induced distortion crosstalk in a lateral direction along a row electrode. As shown in FIG. 15, for example, when the column electrodes X1 through X4 simultaneously transit from an H level to an L level, induced distortion V1 through V4 occurs in some row electrode Yn due to capacities C1 through C4 and resistances R1 through R4 of the row electrode Yn.
In this case, the resistances R1 through R4 are connected in series to the column electrodes X1 through X4, respectively. The magnitude of the above-described induced distortion is gradually increased toward the right side, i.e., the above-described induced distortion crosstalk becomes larger at the further right side of the row electrode Yn as shown in FIG. 15.
In the first conventional technique, the induced distortion crosstalk can be corrected. Such correction is performed in response to a change in voltage on a row electrode. In practice, the correction is only performed on a column-driver-by-column-driver basis where each column driver typically controls about 100 or more column electrodes. For this reason, differences in the induced distortion crosstalk in a lateral direction along a row electrode cannot be completely corrected. Thus, the above-described induced distortion crosstalk cannot be optimally corrected.
The second conventional technique makes an attempt to correct differences in induced distortion crosstalk in a lateral direction along a row electrode by digitally detecting the amount of the correction to be made. In practice, circuit scale is disadvantageously increased so that differences in the above-described induced distortion crosstalk can be corrected and smoothed. In order to perform the correction without an increase in circuit scale, the correction is only performed on a column-driver-by-column-driver basis. The differences in the induced distortion crosstalk in the lateral direction along a row electrode cannot be completely corrected. Similar to the first conventional technique, the induced distortion crosstalk cannot be optimally corrected. Moreover, since the correction is performed every horizontal scanning period, a large error is introduced to an optimal correction.
Further, in the third conventional technique, a correction period equal to one or two horizontal scanning periods is inserted every predetermined number of horizontal scanning periods. Therefore, a small error is only introduced to an optimal correction. However, the set pulse width or pulse amplitude of a correction voltage cannot be changed in small steps. Similar to the first and second conventional techniques, differences in induced distortion crosstalk in a lateral direction along a row electrode cannot be corrected, and therefore the induced distortion crosstalk cannot be corrected.